The present invention relates to a method of manufacturing gradient or distributed index optical elements with the use of a sol-gel process applied to the manufacture of optical elements such as an optical lens.
Conventionally a method of manufacturing gradient index optical elements having gradient index in the diametrical direction, has been carried out using a sol-gel process.
For imparting concentration distribution in the sol-gel process, the methods described in, for example, articles in Journal of Non-Crystalline Solids, 85 (1986)., pp. 244-246 and 100 (1988)., pp. 511-513 as well as U.S. Pat. No. 4,686,195 are well known. In these methods, a sol is prepared by adding an aqueous metal salt solution, that is, an aqueous (CH.sub.3 CO.sub.2).sub.2 PbO solution, as a source of index-modifying Pb cations to make a high refractive index after subjecting the silica sol to a gelling treatment to form a wet gel, this wet silica gel is dipped in a solution capable of dissolving and diffusing the above metal salt, and then the silica gel is dried and sintered.
In the gradient or distributed index optical elements formed in the above conventional manufacturing method, the step of producing a gradient index is performed only one time. In this case, the refractive index at the outer periphery portion is decreased to some extent, but the concentration of the metal salt contributing to the formation of the refractive index distribution is not sufficiently decreased, so that a large value of difference (.DELTA.n) between the refractive index of the center portion and that of the outer periphery portion of the optical elements can not be obtained.
Since an eluting solution producing gradient index distribution to the metal salt in the wet gel does not include metal constituents (ions) present in the gel, it is considered that if the gel is immersed in the eluting solution, as shown in FIG. 2 by a dot-dash line 1, the concentration of the salt at the outer periphery portion of the optical elements becomes zero, theoretically. However, as shown in FIG. 2 by solid line 2, actually, the metal salt concentration does not become zero, and the refractive index at the outer periphery portion exhibits a high value, so that large refractive difference .DELTA.n can not be obtained to the extent to be expected from the theoretical value.